1. Field of the Invention
The invention relates to a method for forming involute or elongate plastic articles from a sheet of thermoplastic material.
2. Description of the Prior Art
Current methods of forming thermoplastic material have certain basic deficiencies which restrict their utility in forming involute or elongate articles. Involute plastic structures, i.e., structures having a neck smaller than an interior cross-section, are generally formed by bonding two or more separately formed pieces of material together, rather than forming the article from a single piece, and the seam or joint required degrades the optical properties of the article. Injection blow molding can be used to form thermoplastic articles, but this method is limited to relatively small articles due to the magnitude of the equipment involved. Extrusion blow molding can also be used to form thermoplastic articles, but the closure on the end opposite the opening forms objectionable marks and loss of optical properties.
The present invention discloses a method for forming elongated and involute articles from thermoplastic material having a variety of novel and innovative features which overcome deficiencies in the methods used to date. The invention provides a methodology for accurate control of the heating of a sheet of thermoplastic material, and the establishment of preselected concentric temperature zones within the thermoplastic material, to facilitate subsequent formation of the articles. The thermoplastic material is rapidly heated so that the material forms easily without unwanted stretching. After the sheet of thermoplastic material is heated, it is placed in a device for forming the thermoplastic material to the desired shape. The heated sheet of thermoplastic material is planarly supported by the device and held in accurately monitored compression to control slippage of the material toward the center thereof. A plug is used to project the center of the sheet of thermoplastic material through an aperture in the planar support, and the material "self-forms" around a sharp corner on the interior of the aperture into an elongated shape. The planar support disclosed herein also provides a means for inducing axial stress in the sheet of material to control the thickness of the thermoplastic material as it is formed from the flat sheet into the elongated piece.
In the formation of involute articles, the elongated piece is formed within an involute female mold. The involute female mold disclosed herein has a flat aperture section forming the entrance to the mold, and a sharp corner is provided at the junction of the flat aperture section and the interior surface of the female mold to eliminate potential formation of stress cracks. The ambient environment of the thermoplastic material as it is elongated is closely controlled to prevent cooling of the material below its forming temperature so that it can be expanded within the mold by injection of pressurized air into the interior cavity of the elongate piece. As the material expands, the concentric temperature zones control relative expansion of different areas of the material so that the final article has controlled and approximately uniform wall thickness.